FIGS. 1A and 1B respectively illustrate optical transmit and receive modules of a type typically used in coherent optical transmission systems. Referring to FIG. 1A, a transmit module 2 typically comprises a connector 4, digital driver 6, analog driver 8, and an optical modulator 10, all of which are mounted on a printed circuit board (PCB) substrate 12. The connector 4 is typically a multi-pin connector which enables digital data signals to be supplied to the module 2 for transmission, as well as electrical power supply and ground connections for the digital and analog drivers. The digital driver 6 is typically provided as a digital signal processor (DSP), for computing digital driver signals based on the received digital data signals. For example, the digital driver 6 may process the received digital data signals to implement an encoding scheme such as Phase Shift Keying (PSK), so that the digital drive signals will take the form of encoded symbols. More complex signal processing functions may be implemented as desired. The analog driver 8 comprises digital-to-analog converters (DACs) and analog signal conditioning circuits (such as power amplifiers, and filters) for converting the digital diver signals into analog driver signals that are suitable for driving the modulator 10. The modulator 10, which may, for example, be a Mach-Zehnder modulator) receives narrow-band optical carrier light from a laser 14, and outputs a modulated optical channel signal based on the analog driver signals. In the illustrated embodiment, the laser 14 is located remotely from the transmit module 2, and the narrow-band optical carrier is supplied to the modulator 10 via an input optical fibre “pig-tail” 16. The modulated optical channel signal output by the modulator 10 is directed to downstream optical devices (such as optical multiplexers etc., not shown) via an output optical fibre pig-tail 18.
Referring to FIG. 1B, a receiver module 20 typically comprises an optical hybrid 22, photodetector block 24, analog receiver stage (A-Rx) 26, digital signal processor (DSP) 28, and a connector 30, all of which are mounted on a PCB substrate 32. The optical hybrid 22 receives an input optical channel signal through an optical fibre pig-tail 34 connected to upstream optical devices (such as an optical de-multiplexer, not shown) and narrow-band light from a local oscillator 36 via a respective LO pig-tail 38. The photodetector block 24 receives mixed light from the optical hybrid, and outputs corresponding analog electrical signals. The analog receiver stage 26 comprises analog signal conditioning circuits (such as power amplifiers, filters etc.) and analog-to-digital (A/D) converters (not shown) for converting the analog electrical signals from the photodetectors into raw digital sample streams which are processed by the DSP 28 to detect and recover digital data signals modulated on the received optical channel signal. The connector 30 is typically a multi-pin connector which enables recovered digital data signals output from the DSP 28 to be supplied to further data recovery and processing systems, as well as electrical power supply and ground connections for the photodetector array 24, analog receiver stage 26, and the DSP 28.
In both of the transmit and receive modules described above, the PCB substrate 12, 32 provides both a structural support for each of the other elements of the module, and the electrical interconnections between them. In the case of the transmit module (FIG. 1a), a digital data bus 40 is provided between the connector 4 and the digital driver 6, which is designed to carry data signal traffic at the intended bit-rate; a high-speed digital interface 42 is provided between the digital driver 6 and the analog driver 8, for conveying encoded digital signals at a desired sample rate. Finally, an analog transmission line bus 44 is provided for carrying the (typically radio frequency) analog drive signals from the analog driver 8 to the modulator 10. In the case of the receive module 20 (FIG. 1b), the optical hybrid 22 and photodetector block 24 are optically connected via optical waveguides 46 which are often supported independently of the PCB substrate 32. An analog transmission line bus 48 is provided for carrying the (typically radio frequency) analog signals from the photodetector block 24 to the analog receiver stage 26. A high-speed digital interface 50 is provided between the analog receiver stage 26 and the DSP 28, for conveying the raw digital sample stream at the A/D convertor sample rate. Finally, a digital data bus 52 is provided between the DSP 28 and the connector 30, which is designed to carry recovered digital data signals at the intended bit-rate.
Typically, the various active components of the transmit and receive modules are provided as separate Integrated Circuit (IC) devices, which are assembled together on the PCB substrate 12, 32, for example using known surface mounting techniques. This arrangement enables each of these components to be separately manufactured (e.g. by different manufacturers) which increases the design freedom in selecting components for each module, and reduces costs. However, this arrangement suffers a disadvantage in that each of the digital and analog buses 40-44 and 48-52 are relatively long, and the impedance of theses electrical interconnections means that each of the active components (principally the digital and analog drivers on the transmit module 2, and the analog receiver stage and the DSP on the receiver module 20) must have suitable impedance-matching and power-driver circuits in order to drive the buses 40-44 and 48-52 and so transmit the required signals. This increases both the cost and power consumption of each of these devices, as well as presenting an additional source of noise. The severity of these problems tends to increase rapidly with increases in either data signal bit rates and complexity of the digital signal processing implemented by the digital driver and DSP components.
For appropriate impedance control at high bandwidths, expensive waveguides and connectors may be required.
Co-pending U.S. patent application Ser. No. 12/721,876 filed Mar. 19, 2010 and entitled Integrated Transmit and Receive Modules, teaches techniques for of addressing this problem, by combining the optical and electronic components into respective Integrated Circuit (IC) elements, which are then connected together (both electrically and mechanically, via solder balls or bumps. Thus, for example, in the case of a receiver, an electro-optical IC includes the optical hybrid 22 and the photodetector block 24, while the analog receiver stage 26 and the DSP 28 are fabricated together in an electronic signal processing IC. The electronic signal processing IC is then connected to a package base (also using solder bumps, for example) which provides an integral pin-connector. A transmitter may be constructed in a similar manner, in which the digital and analog driver stages 6 and 8 are combined in a single electronic IC, which is electrically connected via solder bumps to an electro-optical IC (composed primarily of the modulator 10) and a package base.
An advantage of this arrangement is that the electro-optical IC and the electronic IC can be separately fabricated, and the use of solder bumps to connect to two ICs together eliminates the need for impedance-matching and power-driver circuits in order to drive signals between the two components. However, the use of solder bumps to connect the two ICs together means that they must be fabricated using the same materials, in order to avoid undesirable thermal stress during operation.
For low bandwidth transmission systems, this limitation can be accommodated because the associated (lower) performance requirements mean that the designer has greater latitude for selecting IC materials. However, next generation optical transmission systems are expected to require high bandwidth, low noise, optical modulators and hybrids, and ND converters and DSPs operating at on the order of 100 Giga-Sample per second. In order to achieve satisfactory performance, it is expected that that the electro-optical and electronic signal processing ICs will have to be constructed using materials that are optimized for each function. For example, the electro-optical IC may be fabricated in Indium-Phosphide (InP), and the electronic IC may be fabricated in silicon using Complementary Metal-Oxide Semiconductor (CMOS) technology. However, the use of different materials raises the difficulty in that the two ICs have different, and generally incompatible thermal characteristics. This can make it challenging and expensive to package the two IC components in close proximity to each other.
On the other hand, an analog transmission line connection interposed between the (InP) electro-optical IC and the (silicon) electronic signal processing IC would require a bandwidth on the order of 50 GHz, or higher. Achieving this bandwidth requires very careful control of S21, S11, and S22 parameters of the transmission line out to the desired bandwidth (e.g. 50 GHz), which is also challenging and expensive.
Techniques enabling low-cost interconnection between electro-optical and electronic ICs in high bandwidth optical transmitter and receiver modules remain highly desirable.